This disclosure generally relates to wafer level packaging for integrated circuit packages. Certain aspects of this disclosure pertain to methods and apparatus for treating a partially fabricated semiconductor substrate after a layer of photoresist is patterned, and before material is electroplated into the features on the patterned photoresist.
Integrated circuit (IC) chips or dies are typically interfaced with other circuits using a package that can be connected to, e.g., a printed circuit board (PCB). In some cases, IC packaging includes wafer level packaging (WLP), which is an electrical connection technology that employs relatively large features, typically on the scale of micrometers. WLP is a form of packaging fabricated on the wafer before it is cut into dies. Examples of WLP structures include redistribution wiring, bumps, and pillars.
The packaging standard may be specified industry-wide or may be specific to a particular company or group of companies. For example, an IC die can be packaged as an industry standard ball grid array (BGA) package. A BGA package has an array of solder ball pads positioned on a bottom surface of a substrate, where solder balls contact the pads/ports of a PCB and the solder balls are reflowed to attach to the package of a PCB. In such an implementation of a BGA package, wire bonds are connected between pads/ports of the die and electrical features of the substrate.
Some WLP processes include deposition of a metal seed layer, followed by deposition of a layer of photoresist. The photoresist is then patterned through exposure to light, and portions of the resist are removed to form features (e.g., holes, pads, lines, etc.) in a photoresist development operation. The development process should expose the underlying metal seed layer, which is later used as a conductive surface to initiate electrodeposition within the features. However, some amount of photoresist or other residues will often remain on the seed layer after this development process. The residues are often referred to as “photoresist scum” or more simply “scum.” Photoresist scum may be caused by incomplete removal of photoresist within the features and by the formation of organic residues during the development process. The scum can interfere with later electroplating processes. For example, residues on the portions of the seed layer where electroplating takes place may affect the seed layer's adhesion to subsequently electroplated materials. Further, the presence of residues on the seed layer may lead to poor wetting and additive adsorption during electroplating, which contribute to poor electroplating results.
In many cases, photoresist remnants are removed from the seed layer through a process referred to as “descumming.” Conventional descumming processes involve exposing the substrate to an oxygen-based plasma. In many cases, the gas used to generate the plasma is at least about 90% oxygen. The oxygen-based plasma removes the photoresist scum from the bottom of the features, leaving a resist-free seed layer upon which electroplating can take place. However, conventional descumming processes involve certain drawbacks including potential oxidation of the seed layer. Therefore, improved methods of removing photoresist scum in the context of wafer level packaging processes are desired.